Fine coal, which is a by-product of the coal washing process, often is wasted. It is abandoned in slurry pits or land fills because of the difficulty in transporting and handling. It has been estimated that 28 million tons of coal fines now lie in blackwater ponds. Because it is very fine, when it is dried, it blows as dust. This makes it difficult to transport. When it wets out and freezes it can be impossible to handle. Moreover, once it wets out it does not easily dry out again. My copending U.S. patent application Ser. No. 178,620, filed Aug. 15, 1980 entitled "Method of Recovering and Using Fine Coal" relates to the process for recovering and pelletizing fine coal. This invention relates to another but related process. Very often coal materials are pulverized prior to burning. Hence a process for pelletizing fine coal should produce a pellet that, in addition to being strong and water-resistant, is also easily repulverized in existing equipment. Disclosed herein is a process for producing such a pellet.
One source of fuel that may become very important is lignite coal. Lignite is a mineral substance of vegetable origin which is a geological precursor of coal. It has a much lower heating value than coal, but because of its low sulfur content, may become a very desired fuel for industry. It has certain drawbacks however. It is highly hydroscopic. It takes up and releases water if stored in piles. This process results in crumbling and disintegration which is undesirable and makes shipping difficult. More important, however, the process of loss and gain of water can result in spontaneous combustion which limits the length of time the lignite can be stored. Studies have concluded that lignite cannot be safely shipped for distances of over 100 miles. For that reason, lignite must be "packaged" for shipment and storage. The process disclosed herein enables the pulverization and pelletization of lignite to provide a unique product. Prior pelletization processes have not been applicable to lignite because of its unique physical properties.
Agglomeration or pelletizing of fine particles has been known for a long time. It has also been known that each material has a unique set of conditions under which it is pelletized, if indeed, it can be pelletized. Binders used in pelletizing fine materials have included the following: hydrolized starches, sulfites, mineral oils, and even in some cases only water. Some binders impart sufficient wet strength to agglomerated materials but upon drying do not supply dry strength or survive rewetting.
British Pat. No. 183,430 (1921) teaches agglomerating finely divided carbonaceous material by mixing with water and hydrocarbon oil. The recovery product is an agglomerated mass containing from 10 to 15 percent moisture. The British patent does not suggest that hardened pellets are formed nor would that be expected. The quantities of fuel oil suggested as the hydrocarbon oil would not be compatible with today's fuel oil prices.
U.S. Pat. No. 3,148,140 of Kaiser et al. suggests use of a "water-immissible organic solvent" for recovering of carbon particles from water. The specific solvents named are hydrocarbons, namely hexane, pentane, benzene, toluene, xylene, light naphthas, or mixed paraffin fractions. These volatile materials (boiling range from 35.degree. to 200.degree. C.) can hardly be candidates for oven dried pellets nor is it likely that they would remain upon drying to bind the fine coal. The quantities of solvent suggested are far from economical.
U.S. Pat. No. 3,696,923 by Miller relates to recovery of coal fines through froth flotation. Preferred flotation agents comprise MIBC, pine oil and even fuel oil. The small quantities added are far from sufficient to act as a binder, if indeed, they do.
U.S. Pat. No. 3,043,426 by Noone relates to recovery of fine coal particles by froth flotation utilizing hydrocarbon materials (kerosene and methyl isobutyl carbinol) in very small quantities that are far from sufficient to act as a binder.
A recent study for the U.S. Department of Energy by Babcock Contractors of Pittsburg, Pa. has developed information relating to pelletizing lignite, using an asphaltic emulsion as the binder. The strength of the pellets so made only ranges from 20 to 25 psi. Asphaltic emulsions contain undesirable contaminants such as sodium which may attack boiler tubes in the combustion process.
Having been exposed to the need to produce a strong, weather-resistant coal or lignite pellet, inexpensively, I have concluded that the binder used for such purpose must fuse the particles of carbonaceous materials by reacting with itself and/or with the coal particles. The binder must be fusable at relatively low temperatures and must not, upon drying exhaust noxious or flammable fumes into the atmosphere.
An essential aspect of this invention is a drying step, as in an oven, to remove moisture. Fine coal and lignite requires water to agglomerate to a suitable size. The water serves as a temporary binder but it is not desirable in the end product. Moreover, in certain embodiments of the process large amounts of water are associated with the fine coal either because it is being recovered from blackwater ponds or because the fines are slurried with the binder to achieve the desired coating. But, water in the final product is undesirable as it is useless weight, increasing transportation costs per BTU and decreasing the BTU content per ton.
Prior to my work, described in the above identified copending patent application, the suggested binders for coal fines were principally petroleum based hydrocarbons. No thought was given to fusing probably because the temperatures required would have resulted in ignition. Attempts by me to fuse coal fines with engine oil at temperatures below ignition temperature have produced only a weak pellet that crumbles with finger pressure.
Starches and sulfites have no apparent ability to completely fuse the pellets of coal fines. Moreover, when rewetted, the bond weakens. Thus the pellets made with starch and sulfide binders are neither strong nor waterproof. Sulfites also add sulfur to the coal which produces undesirable sulfur oxides in stack gases.
As disclosed in my copending application referenced above, fixed oils (vegetable oils as they are sometimes called) are extremely useful in agglomerating and fusing coal fines. Vegetable oils comprise unsaturated fatty acids (and esters thereof) which fuse when heated in air. They also have an affinity for coal surfaces in a water slurry such that when coal is slurried with an excess of vegetable oil, the coal particles only take up the amount of oil necessary for good fused particle binding, i.e., 3 to 5 percent by weight. Typically, these pellets have strength in excess of 40 psi. Vegetable oils are less expensive than petroleum oils especially when use is made of waste materials. In searching for additional sources of fixed oils, I have discovered a binder material that has as much as three times the strength of vegetable oil binders and six times the strength of asphaltic binders. Moreover, the agglomerates made using this binder are easily pulverized due to the strong but brittle nature of the binder. While vegetable oils have been found to work extremely well with most coals, it has not been found to work with lignite. The binder material disclosed herein is suitable for packaging lignite for shipment, i.e., producing a water-resistant lignite pellet.